1. Field of the Invention
The present invention relates to an optical transmission device, a scrambling method, and a descrambling method, and, more particularly to an optical transmission device, a scrambling method, and a descrambling method that enable to mix and transmit signals of different bit rates at a low cost.
2. Description of the Related Art
In recent years, attention is being focused on optical transmission systems using optical fibers as a communication system having the potential for large volumes of data at high speed. In an optical transmission system, an optical line terminal (OLT) serving as a host station device and a plurality of optical network units (ONUs) functioning as subscriber devices are connected by optical fibers.
In an optical transmission system, which is generally called a passive optical network (PON), various studies are being performed for further increasing transmission capacity. For example, in a technology disclosed in Japanese Patent Application Laid-open No. H9-214541, a provider device includes framers that correspond to a plurality of ONUs and the respective framers transmit frames of different transmission speeds to the ONU. The technology mentioned earlier enables to individually set bit rates of the frames that are transmitted to each ONU and enables to connect a single provider device with the multiple ONU of different transmission capacities. Due to this, in the optical transmission system, the transmission capacity can be gradually increased while enhancing performance of a portion of the ONU.
However, in the technology mentioned earlier, the multiple independent framers are included in the single provider device, thus significantly increasing the cost. Further, the technology mentioned earlier presupposes a single star (SS) type as a network topology. Due to this, an existing network form needs to be significantly changed, thus increasing the cost.
In a method for gradually increasing the transmission capacity at a low cost, speeds of the bit rates in data of the frames are increased and speeds of the bit rates in a header of the frames are matched to the bit rate of the lowest speed. In other words, in the frame which is addressed to the ONU of a high transmission capacity, causing the bit rates in the header and in the data to differ and increasing a capacity of the data enables to increase the transmission capacity. Further, for all the frames, because the bit rates in the header are equal to the bit rate of the lowest speed, all the ONU can check contents of the frame in common and all the ONU can determine from the header whether the data is addressed to the respective ONU itself.
Thus, the bit rate in the header of the frame is common to all the ONU and the bit rates in the data of the frame vary according to the respective ONU. Due to this, the provider device in the form of an optical line terminal (OLT) transmits the frame in which a plurality of the bit rates are mixed. A method needs to be devised to perform a scrambling process on the frame of the mixed bit rates. However, the scrambling process on the frame of the mixed bit rates has not been studied yet.